The invention relates to a yarn feeding device including a stationary storage body and a yarn brake incorporating a braking body which is held axially against the storage body, and a yarn braking body for a yarn feeding device.
The braking body of a yarn brake of a yarn feeding device known from EP-A-06 86 128, well known in the market under the name xe2x80x9cflex brakexe2x80x9d, comprises an annular rubber or plastic material membrane of circumferentially constant wall thickness. Arc-shaped bending zones are provided between the holding area of the braking band and the carrier ring. The holding membrane has an annular skirt which grips over the rear side of the braking band which is glued to the skirt. When mounted at the yarn feeding device the yarn braking body braking band is held by axial pre-tensioning force against a rounded withdrawal rim of the storage body. When withdrawn the yarn runs over the withdrawal rim and below the braking band which defines a circumferentially continuous braking surface and has a generally line-shaped contact area on the withdrawal rim. At the location of the yarn passage the braking band is lifted from the withdrawal rim and forms a sickle-shaped intermediate space. Since the withdrawn yarn fulfills an orbiting motion about the withdrawal rim, the sickle-shaped intermediate space moves with the yarn. The yarn additionally actuates the braking band by a friction force resulting in a slight local deformation of the braking band such that the basically unstretchable but elastically bendable braking band may form waves. The holding membrane transferring the axial pre-tensioning force into the braking band follows the operational movements of the braking band. From the local deformations of the braking band the contact pressure at the withdrawal rim of the storage body may vary leading to an irregular behaviour of the yarn tension in the withdrawn yarn. This influences the self-compensation performance of the yarn brake. The self-compensation performance means that the braking band automatically decreases the braking effect when the yarn tension increases by strong yarn acceleration, and vice versa increases the braking effect when the yarn tension automatically decreases due to a strong yarn deceleration, in order to maintain a uniform yarn tension profile.
In one embodiment (FIGS. 14, 15) of a yarn brake for a yarn feeding device as known from WO-A-98/38124, the braking band is floatingly suspended at its rear side on a conical elastic ring body. In the contact area of the ring body, circumferentially consecutive elevations and depressions may be provided. In the embodiment shown in FIG. 16 the braking is held at its outer edge region within a circumferentially extending pocket of an elastic annular holding membrane which is connected outside of the holding area to a carrier ring arranged concentrically with respect to the braking band. Between the carrier ring and the holding area of the braking band, the holding membrane has a circumferentially continuous wall thickness.
It is an object of the invention to improve a yarn feeding device as mentioned above as well as a yarn braking body such that in operation a tension profile can be achieved in the withdrawn yarn, which tension profile is as uniform as possible, and that undefined deformation actions are minimized in the braking band.
The above object is achieved by providing an elastic holding membrane which holds the braking band of the braking body by its outer edge region, and the holding membrane has a plurality of area portions with a reduced wall thickness between a carrier ring and the area of the holding membrane which holds the braking band.
Unexpectedly, the support of the braking band in the holding membrane is improved by the area portions each having a reduced wall thickness. Spring spokes are defined between adjacent area portions, and the spring spokes mainly transmit the forces from the carrier ring into the braking band. The spring spokes may be formed with the normal wall thickness of the holding membrane. The wall thickness in the area portions is significantly smaller. The area portions of reduced wall thickness form intermediate spaces between the spring spokes. At least adjacent to the holding area, the intermediate spaces are filled out by swim skin like membrane layers. The area portions may have a randomly selectable shape and occupy a selectable part of the overall usable area. The wall thickness of the holding membrane or its degree of hardness may be higher than in the known holding membrane such that the holding rigidity for the braking band is higher and improves the centering of the braking band on the withdrawal rim. During withdrawal of the yarn, a controlled deformation of the holding membrane between the holding area and the carrier ring is achieved by means of the area portions. Undefined deformations of the braking band otherwise resulting in irregularities of the yarn tension are compensated for or are dissipated. During the orbiting motion of the yarn, the area portion situated close to the withdrawal point takes up forces. The area portion gets deformed, e.g. by buckles or depressions, and assists in avoiding undefined deformation of the braking band. The yarn tension profile in the withdrawn yarn is made more uniform. The braking band operates in a desirably damped manner since the area portions of reduced wall thickness result in a lazy response behaviour of the holding membrane. The important self-compensation performance is assured even with the highest yarn withdrawal speeds and for difficult yarn qualities. Thanks to the swim skins in the area portions, no openings are formed in which lint would otherwise deposit or the yarn would be caught, particularly when being threaded through the yarn brake. Finally, manufacturing of the holding membrane and of an injection molding tool are simpler if in view of a predetermined deformation performance between the holding area and the carrier ring and if the area portions influencing the deformation behaviour are not formed open but with reduced wall thickness.
In circumferential direction the area portion may be distributed regularly.
In order to achieve a soft, relatively damped spring property of the holding membrane the holding membrane ought to be formed with at least one bend between the holding area and the carrier ring. In this bend the spring spokes are acting like bending springs transmitting the forces to the braking band.
Expediently, the circumferential widths of the area portions are different from the circumferential widths of the spring spokes, and are preferably smaller. However, it is possible to provide about the same circumferential widths for the area portions and the spring spokes.
The ratio between the wall thickness in the area portions and the wall thickness of the spring spokes should be about 1:5.
Expediently, the area portions as well as the spring spokes are oriented substantially radially. Said orientation allows use of the yarn braking body irrespective of the withdrawal direction of the yarn from the storage body for each withdrawal direction. Furthermore, said orientation results in an efficient centering of the braking band on the storage body, irrespective of the yarn withdrawal direction.
The area portions may have a substantially oval shape. This also is of advantage for manufacturing the braking body.
The area portions and the spring spokes should define an essentially smooth surface at the convex side of the bend. Said surface facing the incoming direction of the yarn assures that no lint is collected and that the yarn is not caught when touching the surface. The area portions having the reduced wall thickness may be under pre-tension in the surface, i.e. so that the thin membrane layer bridging the intermediate space so to speak is in a stretched condition in one or several directions.
Expediently, the bend is prolongated inwardly by a conical skirt having smooth surfaces. The skirt supports the braking band at its rear side and damps an excessively lively spring behaviour of the braking band.
A protruding lip may be formed in the holding area. The lip defines with the skirt an open insertion pocket for the braking band. The braking band may be glued to the skirt. In most cases it is sufficient to only insert the braking band into the insertion pocket and to loosely secure it in operative position exclusively by the form fit. In operation the position of the braking band anyway is fixed by the contact pressure of the braking body and the contact between the braking band and the skirt.
Said lip may protrude by a distance amounting to about 10% to 25% of the width of the braking band. Said dimension facilitates mounting or inserting or removing the braking band and nevertheless assures a proper positioning of the braking band, even during transport of the braking body.
The area portions having the reduced wall thickness should begin a short distance outside of the insertion pocket in which the braking band is positioned by its outer edge.
The wall thickness of the skirt may be smaller than the wall thickness of the spring spokes but larger than the wall thickness in the area portion.
The holding membrane expediently is a unitary injection molded part made from a tough elastic plastic material, such as polyurethane. The spring spokes which are important for the function of the holding membrane and act one by one due to the provision of the softer interspaces can be formed by simple tooling if the intermediate spaces are bridged by thin layers of the material of the holding membrane. Said design significantly simplifies the manufacturing process of the holding membrane by injection molding and leads to the advantage that the intermediate spaces, thanks to the swim skins, participate in the energy distribution or damping energy dissipation and support the braking band.